1. Technical Field
The present invention relates to the field of the technologies associated with the measurement and the tracking of the direction of gaze (known as “eyetracking”) and relates, more particularly, according to a first of its aspects, to a method for tracking the gaze of a user on a display surface observed by the user, consisting notably of the screen of a mobile terminal such as a mobile telephone or a touch-screen tablet, allowing it to be determined, while mobile, at which locations on the observed display surface the gaze of the user is aimed.
2. Related Art
By way of non-exclusive example of an application of the invention may notably be mentioned the analysis of the interactions of the gaze with a user interface, undertaken in the framework of behavioral studies aimed for example at evaluating areas of interest within this interface or at adapting the ergonomic properties of this interface.
In the prior art, two large families of eyetracking systems are distinguished:                non-intrusive and non-portable systems;        intrusive and portable systems.        
All these systems use a basic technical principle consisting in evaluating the direction of gaze starting from the relative position of the corneal reflection of a light source and of the pupil.
The principle consists in disposing an LED (light-emitting diode) as a light source emitting radiation in the infrared wavelengths right next to the objective lens of a video camera intended to acquire images of the eye. This infrared LED emits light toward the eye in the same direction as the axis of the objective lens of the video camera. This light is reflected on the cornea of the eye: a light spot, called “Purkinje spot”, then appears on the image of the cornea of the eye captured by the video camera.
The non-intrusive tracking system is a system which allows the movements of the eye to be recorded and hence the gaze to be tracked when the subject is for example looking at a computer screen.
This type of system is generally composed of a very high definition camera disposed at around sixty centimeters from the head of the user in the axis of the gaze of the subject.
Since this camera is high definition, it is then possible to distinguish the face of the user toward whom it is pointing. Then, by image processing and shape recognition, it is possible to identify the location of the eyes and to enlarge the image in order to accurately determine the position of the pupil with respect to the Purkinje spot. This system does not interact with the user, and the latter does not have to wear special glasses or a helmet.
However, the main drawback of this system is that it relies on the use of a high-definition camera whose cost exceeds by more than 100 times the price of a camera with standard resolution. In addition, during the analysis of the gaze, the user cannot move his/her head beyond a three-dimensional limit defined by the angle of view of the camera which is filming the eye.
Furthermore, the screen that the user is watching must necessarily be fixed, which constitutes an important limitation, preventing this type of system from being adapted to display screens of mobile terminals such as mobile telephones or touch-screen tablets for example, which, by their nature, are used while moving.
Intrusive eyetracking systems allow this eyetracking to be recorded when the user is looking at a global scene such as the cockpit of an aircraft for example or else the dashboard of a car.
This type of system is generally composed of a first video camera disposed at around ten centimeters from the head of the user in the axis of gaze of the user. This video camera is close to the head, and it must therefore be fixed to a special pair of glasses or to a helmet.
Since this video camera is directly filming the area of the eye, by image processing, it is possible to accurately recognize the position of the pupil with respect to the Purkinje point.
Thus, this system endows the user with a great freedom of movement. The user can thus move around while at the same time affording the camera the possibility of analyzing the direction of his/her gaze. In addition, such a system allows the use of all types of cameras having a reduced cost compared for example with high-definition cameras.
This type of system also comprises a second video camera, referred to as a ‘scene camera’, for filming the scene observed by the user. The two video streams respectively generated by the first video camera and by the second video camera are processed in order to produce a single video stream representing the scene and including a cursor allowing the locations in the scene where the user directs his/her gaze to be tracked at each moment in time. Such a system is well known to those skilled in the art, notably by the example given in the document US 2010/0053555.
However, one drawback of this system resides in the fact that the first video camera used to determine the position of the eye and the second video camera allowing the scene observed by the user to be filmed are in the same reference frame, which is linked to the reference frame of the head of the user. Moreover, this system is not well adapted to tracking the gaze over the surface of a mobile object, and, more particularly, over the display screen of a mobile terminal, such as a mobile telephone, in a context of mobility. Indeed, when the scene camera records a scene where the user is in the process of looking, while moving around, at the screen of a mobile telephone for example, the object in question being watched and filmed becomes mobile in the stream of video images resulting from the recording of the scene by the scene camera. It is then particularly difficult to be able to automatically determine what the user, while moving around, is looking at.
For example, if the user is continuously looking at an area of interest on the screen of the mobile telephone in a context of mobility, the point of gaze of the user does not move according to said area of interest but, on the stream of video images resulting from the recording of the scene by the scene camera, it moves around according to the position in the image of the area of interest being watched, itself mobile within the resulting video stream.
Accordingly, within this context of mobility, it is not possible to readily, and especially automatically, determine that the user is continuously looking at the area of interest in question, except, which is not desirable, by endowing the system with powerful image processing and shape recognition means, capable of accurately recognizing the area of interest in question within the stream of video images, in order to deduce from this that the point of gaze on the image really has followed this area of interest, mobile within the image.
In this context, the aim of the invention is to overcome the drawbacks of the prior art and, in particular, to solve the problems encountered by the known eyetracking systems of the intrusive and portable type in order to be able to readily and automatically determine, within a context of mobility, the direction of gaze with respect to an object being watched, notably a mobile telephone.